The Global System Mobile Communications (GSM) is used by over 80% of the global mobile market, that is, it is used by over 2 billion users in more than 212 countries and territories. GSM is a substantial improvement over its predecessors in that both signaling and speech channels are digital call quality. GSM is considered a Second Generation (2G) mobile communication system. The newer 3rd Generation Partnership Project (3GPP) networks and 3GPP-enabled mobile communication devices are capable of transmitting and receiving substantially higher data throughput than 2G. Current 3GPP standards incorporate the latest revision of the GSM standards. On the other hand, the other 20% of the global mobile market utilizes Code Division Multiple Access (CDMA) which specifies standards for another 3G technology, 3rd Generation Partnership Project 2 (3GPP2). Both standards continue to evolve.
For data transfer under a GSM air interface, General Packet Radio Service (GPRS) provides a packet oriented (digital) Mobile Data Service. GPRS may be used for data transfers such as Wireless Application Protocol (WAP) access, Short Message Service (SMS) Multimedia Messaging Service (MMS) and for Internet communication services. GSM and GPRS networks both employ Gaussian filtered Minimum Shift Keying (GMSK) as a modulation scheme because of its narrow bandwidth, robustness to fading and phase noise as it has a low modulation level and may employ a high number of redundancy bits, and relatively low implementation complexity. GMSK's modulation carries one bit per channel symbol. For higher data throughput, a data capacity improvement to GPRS is the Enhanced General Packet Radio Service (EGPRS) which squeezes more data carrying capacity out of the GSM air interface than GPRS. In addition to GMSK, EGPRS uses Eight Phase Shift Keying (8-PSK) modulation which has three times the bit-per-symbol density of GMSK. The drawback is that 8-PSK is less robust than GMSK.
Nine different Modulation and Coding Schemes (MCSs), identified as MCS-1 through MCS-9, are defined by the current EGPRS system specifications. EGPRS networks utilize both GMSK and 8-PSK. MCS-1 through MSC-4 are GMSK. MCS-4 through MCS-9 are 8-PSK. As mentioned above, GMSK is more robust than 8-PSK. Moreover, the higher the MCS number, the less reliable the coding scheme. For example, MCS-4 is less robust than MCS-3, and so on.
3GPP has also released a new version of EGPRS called EDGE Evolution. With EDGE Evolution, latencies are reduced and bit rates are increased up to 1 MBit/s. Higher symbol rate and higher-order modulation (32 quadrature amplitude modulation, 32QAM, and 16QAM instead of 8-PSK) are used with new MCS called UAS and UBS (EGPRS2 Uplink level A modulation and coding Schemes and EGPRS2 Uplink level B modulation and coding Schemes). The same remark on robustness applies to UAS and UBS coding schemes.
In 2G networks, when data needs to be transferred from the mobile station (MS) to the network (NW), the MS initiates the establishment of an uplink Temporary Block Flow (TBF). The NW will allocate radio resources to be used by the MS to transfer uplink data. There are several variants of this procedure. A typical scenario is the following (known as “two phases access”). The MS first sends a CHANNEL_REQUEST or PACKET CHANNEL REQUEST for resource allocation. The NW allocates in return limited resources (1 or 2 radio blocks). The allocated radio resources are transmitted to the MS in a PACKET UPLINK ASSIGNMENT or IMMEDIATE ASSIGNMENT message. The MS transmits a PACKET RESOURCE REQUEST message to the network by using the limited allocated radio resources. The network may then assign radio resources for the uplink TBF with a PACKET UPLINK ASSIGNMENT message. In the PACKET UPLINK ASSIGNMENT message, the network indicates the MCS to be used by the MS to transmit data blocks in uplink. This MCS is referred as the “commanded MCS”. In another scenario, known as “one phase access”, the NW directly assigns the radio resources to the MS when receiving the CHANNEL REQUEST, without the limited resource assignment/PACKET RESOURCE REQUEST sending step.
In the current EGPRS standards, the CHANNEL_REQUEST and PACKET_CHANNEL_REQUEST are encoded using GMSK modulation and a robust coding scheme. Similarly, under standards, the PACKET RESOURCE REQUEST message, in the case of two phase access, is encoded using GMSK modulation and a robust coding scheme. However, as soon as network allocates radio resources for the uplink TBF, the MS uses the MCS commanded in the PACKET UPLINK ASSIGNMENT message to transmit the uplink data. Indeed, an EGPRS network commands the type of Modulation and Coding Scheme (MCS) that the mobile communication device will use to transmit data to the network. In particular, the EGPRS network may command that the mobile communication device transmit data at the highest rate that the network is capable of receiving and processing, e.g. using a less robust 8-PSK MCS.
However, since the initial request messages (CHANNEL_REQUEST, PACKET_CHANNEL_REQUEST, PACKET RESOURCE REQUEST message) are encoded using GMSK modulation and a robust coding scheme, whereas the uplink data using the commanded less robust MCS, it can happen that the network is not able to decode the uplink data even if it has correctly decoded the initial access request messages.
Failure of uplink data transmissions using a less robust MCS may occur for different reasons. In particular, a mobile communication device may be located near the boundary of its serving network cell (typically, the MS may be far from any NW radio stations). Also, a mobile communication device may cross from one network cell range into another network cell range. Additionally, there may be some type of signal disturbance or interference. Therefore, the failure to transmit in accordance with the commanded MCS may be self-rectified when the mobile communication device roams into an optimal range or a signal disturbance ceases. Also, the NW may command an 8PSK MCS and ask for the maximum output power from the MS. Since typically the maximum output power of the MS is lower in 8PSK than in GMSK, this can increase the probability of having the uplink data not decoded by the NW.
The mobile communication device will continue to attempt to transmit data in the commanded MCS until it receives acknowledgement from the network that the data has been received, or a non-acknowledgement message indicating which data blocks have not been received by the NW. In such message, NW could assign a different “commanded MCS” more robust than the initial one, in order to adapt the uplink transmission to the radio conditions (this is known as link adaptation). However, receiving such a message can take some time. This is dependent on the NW implementation. Typically the NW could wait for receiving some data from the MS before sending such a message. In case of bad radio conditions, the NW may hardly notice that the MS has started transmitting with the commanded MCS, and as a result may either delay the sending of a non-acknowledgement message, or may not even send any non-acknowledgement message. Since the MS must use the less robust commanded MCS to transmit uplink data, there may be no way to improve the uplink transmission. Even the sending of a small amount of data (which is typically the case for mobility signaling procedures for instance) can fail. Impacts are multiple. While repeatedly failing to successfully transmit data to the EGPRS network in accordance with the commanded MCS, the current drain of the mobile communication device may be substantial, particularly if finally receiving the data of the transfer takes a substantial amount of time. Moreover, with repeated failed attempts service availability suffers. For example, a user may be unable to receive circuit switched calls during such attempts. Moreover, if the uplink data transfer is linked to a mobility signaling procedure, impact may be worse since the MS will try several times to perform this procedure, and may finally have to change its serving cell. An improvement in the uplink data transmission procedure may be beneficial.